This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. High-field ESR spectra offer considerable improvements in orientational resolution over low field spectra, in many cases. Recognizing this, we have now successfully extended our capabilities to single-crystal rotation studies of Kramers and non-Kramers transition metal ions at high fields. Our design was influenced by single and multi-axis goniometers at W-band. However, we have incorporated a number of features into our design, which are specifically tailored to facilitate operation at low temperatures in a Fabry-P[unreadable]rot resonator. Multifrequency studies of single-crystal rotations allow one to more precisely determine the orientation-dependent terms in the spin-hamiltonian, such as the g-matrix, hyperfine-matrix and the zero-field splitting (ZFS) tensor. At sufficiently high fields, high-field spectra of single crystals with their narrower intrinsic linewidth compared to powder spectra may be used to assign the diagonal ZFS terms with confidence. At lower fields, the off-diagonal terms in the ZFS tensor become significant. This information, along with careful modeling of the ESR active moiety, can be used to obtain important structural information. Single-crystal studies of metalloenzymes with large ZFS splittings will particularly benefit from a multifrequency approach. Given the low or imperfect symmetry of many important biological systems, it is important to develop a goniometer that will work smoothly in a multi-frequency spectrometer with a multi-axis capability, as well as have efficient operation at low-temperatures for systems with short relaxation times. An improved version of the single-axis goniometer we have been using hitherto, featuring higher precision, is nearly completed and will serve as a test bed for developing a high precision multi-axis goniometer. We will incorporate this higher precision goniometer into our newly developed broadband bridge with improved temperature control and higher sensitivity to facilitate the study of a variety of biologically relevant single crystals.